We show using a minimalistic theoretical framework that phase transition decoupling in lipid bilayers is caused by a nonuniform stress profile due to an asymmetric distribution of lipids between the two leaflets. Applying this framework to vesicles, we demonstrate that their anomalous freezing is also caused by a stress asymmetry, but that this is due to lipid tail extension on freezing. Finally, we predict a previously unknown dependence of surface tension on temperature, find the phenomenon also in coarse grained molecular dynamics simulations, and suggest that it might have relevance in thermosensitive protein gating.